How does an XMP file store vignette info?

I'm having issues in timelapse sequences with vignette properties changing when I change shutter speed and I need to vary it via the XMP data.  I set keyframes, edit those, then interporlate the values in the XMP files via a custom tool.  There are thousands of images so it's not practical to do it manually - the code interpolates the values set in the keyframes.

The issue is I can't figure out how the XMP file stores vignette data.  There's a slider in Camera Raw for "Vignette" but there's no corresponding entry in the XMP file.  There's a "VignetteModelParam1" but it's always equal to 1.  Then there's a collection of entries under "VignetteModelPiecewiseParam" but those values change with every image, even the ones where the slider in Camera Raw is unchanged.  So I don't understand what those values represent.  It looks like they represent some kind of a falloff value as a function of radius from the center of the image but why do they change for images where the vignette slider is in the same location (100)?  There's something else going on there.

So... does anybody know how Adobe stores vignette info in the XMP file?

Thanks in advance,

rgames